In the conventional method of making high tension ignition cable, there is provided a center tension member upon which the remaining layers are fixed. The tension member is passed through an extrusion device and a plastic layer is applied thereto. This layer may contain ferromagnetic material such as ferrite powder. The tension member is made of materials having a high tensile strength.
The materials of which the plastic layer is composed are silicone rubber, chlorinated polyolefinic elastomers, including chlorinated polyethylene, and the like. After being extruded over the tension member, they are cross-linked at elevated temperatures and pressures.
Once the foregoing is accomplished, a wire, usually a resistive conductor, is coiled around the cross-linked plastic layer. Thereafter, an insulation layer, a braid, and a plastic sheath are applied successively to the cable.
In recent times, it has been found desirable to increase the inductance of the cable per unit length. In order to accomplish this, it is important that the coils of the wire or resistive conductor be wound more closely around the plastic layer in order to provide a greater number of turns.
However, a problem has arisen in this regard. It has been found that the coils of resistive conductor are easily deformed by the extrusion of the insulation layer; this results in variations in coil alignment and, in some cases, produces actual contact between-adjacent coils. This, of course, makes it very difficult to maintain the desired design inductance throughout the cable length.
There have been attempts at solving this problem. For example, Japanese Utility Model Unexamined Publication No. 146,812/84 teaches a coil configuration wound around a crosslinked plastic layer having fin-like portions which, project outwardly from, and extend longitudinally of, the cable surface. The combination of very tight coil winding and the aforementioned fins are relied on to prevent or minimize the undesired movement of the coils.
In Japanese Patent Unexamined Publication No. 106,884/79, the resistive conductor is wound tightly over a heated, softened surface of the plastic layer and thereby embedded therein. The coils are maintained under tension until the insulation surface is chilled.
Neither of the foregoing were successful in achieving an unchanged coil structure after extrusion of the sheath elastomer. In the first case, an extremely high tension on the resistive conductor is necessary in order to obtain a rigid coil structure. This, of course, causes breakage and creates other problems.
In the second case, the coil structure is also non-uniform, but for a different reason. It is not feasible to uniformly soften the elastomer surface so that the embedding of the coils takes place evenly.
As a result of the lack of stability of the prior art cables, it is difficult to make oridinary cable connections between segments thereof. It is desirable to make such connections by simply removing the outer layers (e.g. the insulating layer, braid, and sheath) from the core without disturbing the coil structure.